This invention relates generally to an electromagnetic flowmeter whose electromagnet is excited by an alternating or pulsatory current having a predetermined drive frequency to produce an output signal indicative of flow rate, and more particularly to a system in which noises in the signal rising from harmonics of the drive frequency are suppressed.
In a conventional electromagnetic flowmeter, the fluid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an electromagnet. When the fluid intersects this field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the fluid and hence to its average volumetric rate, is then amplified and processed to yield an output signal for actuating a recorder or indicator, or for carrying out various process control operations.
The magnetic field may be either direct or alternating in nature, for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the D-C signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
Though A-C operation is clearly advantageous in that polarization is obviated and the A-C flow-induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications. The two spurious voltages that are most troublesome are stray capacitance-coupled voltages from the coil of the electromagnet to the electrodes, and induced loop voltages in the input leads. The electrodes and leads in combination with the liquid extending therebetween constitute a loop in which is induced a voltage from the changing flux of the magnetic coil.
The spurious voltages from the first source may be minimized by electrostatic shielding and by low-frequency excitation of the magnet to cause the impedance of the stray coupling capacitance to be large. But the spurious voltage from the second source is much more difficult to suppress.
The spurious voltage resulting from the flux coupling into the signal leads is usually referred to as the quadrature voltage, for it is assumed to be 90.degree. out of phase with the A-C flow-induced voltage. Actual tests have indicated that this is not true in that a component exists in-phase with the flow induced voltage. Hence, that portion of the "quadrature voltage" that is in-phase with the flow-induced voltage signal constitutes an undesirable signal that cannot readily be distinguished from the flow-induced signal, thereby producing a changing zero shift effect.
Pure "quadrature" voltage has heretofore been minimized by an electronic arrangement adapted to buck out this component, but limination of its in-phase component has not been successful. Existing A-C operated electromagnetic flowmeters are also known to vary their calibration as a function of temperature, fluid conductivity, pressure and other effects which can alter the spurious voltages both with respect to phase and magnitude. Hence it becomes necessary periodically to manually re-zero the meter to correct for the effects on zero by the above-described phenomena.
All of the adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field (d.phi.)/dt, serving to induce unwanted signals in the pick-up loop. If, therefore, the rate of change of the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent. Zero drift effects would disappear.
When the magnetic flux field is a steady state field, as, for example, with continuous d-c operation, the ideal condition d.phi./dt=0 is satisfied. But d-c operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered, as previously explained. In order, therefore, to obtain the positive benefits of a steady state field without the drawbacks which accompany continuous d-c operation, the U.S. Pat. No. 3,783,687 to Mannherz et al. discloses an excitation arrangement in which the steady state flux field is periodically reversed or interrupted. The entire disclosure of this patent is incorporated herein by reference.
In the Mannherz et al. patent, in order to avoid the spurious voltages which result from stray couplings without, however, causing polarization of the electrodes, the electromagnet is energized by a low-frequency square wave. This wave is produced by applying the output voltage of an unfiltered full-wave rectifier to the electromagnet and periodically reversing the voltage polarity at a low-frequency rate by means of an electronic switch.
Since the steady state field produced by the square wave is disrupted by switching transients occurring at the points of reversal, the converter to which the signal from the electrodes is applied includes a demodulator which is gated synchronously with the electronic switch to yield an output signal only when the magnetic flux achieves a steady state condition.
While this Mannherz et al. system avoids spurious voltages, it fails to take into account harmonic noise. Because the excitation current for driving the electromagnet has a predetermined frequency, the flow-induced signal yielded by the electrodes contains harmonic noise components which are even and odd harmonics of the drive frequency. These noises result in a less favorable signal-to-noise ratio and impair the reliability and efficiency of the flowmeter.